1. Field of the Invention
This invention is directed to a method for manufacturing ducts which require at least one smooth surface and load bearing stringers or the like and more particularly to an inexpensive method for forming such ducts for aircraft engine purposes.
2. Description of the Prior Art
For years, ducts of this type have been made from sheet and stringers, honeycomb sandwich and chem-milled structures.
In the sheet and stringer types, a sheet metal shell forms the shape of the duct and the sheet metal is then stiffened by load bearing stringers which are riveted or brazed to the sheet metal shell.
In the honeycomb sandwich core reinforced duct structures, honeycomb core stabilizes the thin sheet metal facings by sandwiching it between smooth inner and outer metal liners which form the duct and carry the loads required thereof.
More recently, ducts have been constructed by chem milling a formed structure so that ribs are formed and exposed in an isogrid pattern and the ribs are made very thin for weight reduction. The ribs in this structure, like the stringers of the first mentioned structure, carry the required loads while the thin web metal sheet areas establish the shape and contain the relatively low pressure gas path.
In nearly all methods of manufacture with the exception of the more expensive honeycomb sandwich structures, ducts of this type derive their strength capability through the stringers, ribs or bead configurations The shell or sheet metal part of the structure serves mainly to form the duct configuration and confine the desired gas path.